1. Field of the Invention
This invention relates generally to earth-boring drill bits and particularly to improved head sections for such bits.
2. Background of the Art
In drilling bore holes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cutters are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface, or the bit is rotated by downhole motors or turbines. The cutters or cones mounted on the bit roll and slide upon the bottom of the bore hole as the bit is rotated, thereby engaging and disengaging the formation material to be removed. The rolling cutters are provided with cutting elements that are forced to penetrate and gouge the bottom of the borehole by weight of the drillstring. The cuttings from the bottom sidewalls of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow drillstring.
Before the cuttings are washed away, the cuttings slide over portions of the drill bit while the bit is rotating. The cuttings are abrasive and can cause wear on the surfaces of the drill bit, which can eventually lead to failure. When faced with wear problems, especially in the art of the cutting elements on the cutters, it has been common in the arts since at least the 1930s to provide a layer of wear-resistance metallurgical material called “hardfacing” over those portions of the teeth exposed to the most severe wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast, or macrocrystalline tungsten carbide, dispersed in a metal matrix. Such hardfacing materials are applied by welding a metallic matrix to the surface to be hardfaced.
Moreover, sometimes the cuttings accumulate and get compressed between the cutters and the bit legs that support the cutters or cones. In these situations, the abrasive cuttings can damage the seals that are positioned between the cutters and the bearings that hold the cutters relative to the bit legs of the drill bit. A rounded end of the bit leg that corresponds with the cutter is commonly referred to as a shirttail. Various attempts have been made in differing the geometry of the shirttail in order to reduce the ability of cuttings to accumulate between the cutter and the bit leg. For example, designers have extended the shirttail to slightly overhang the gap between the cutter and the bit leg. However, as the lifespan of the cutters continues to grow, cuttings continue to accumulate, becoming lodged with time, and eventually damaging and causing failure of the bearing seals.